Endrin metabolism studies in endrin susceptible and resistant pine mice have shown one major fecal metabolite, F-l, and two urinary metabolites, U-l and U-2 are produced by both strains. The susceptible strain also produced a third urinary metabolite, U-3. The resistant animals produced twice as much F-l and a l.6-fold greater amount of U-2. The excretion of U-l was equal between strains. In in vitro studies, endrin was enzymatically metabolized to a specific metabolite, L-l, with the resistant strain showing the greatest ability to produce L-l. Further studies revealed that F-l, U-2, and L-l were the same metabolite, the anti isomer of l2-OH-endrin.